The present invention relates to a method and apparatus using electrochemical pretreatment for analysis of a liquid sample stream (e.g. by ion chromatography).
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,956,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin bed. A different form of suppressor column is described and published in U.S. Pat. No. 4,474,664, in which a charged ion exchange membrane in the form of a fiber or sheet is used in place of the resin bed. The sample and eluent are passed on one side of the membrane with a flowing regenerant on the other side, the membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
Another form of suppressor is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site to site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant. The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
One problem with ion chromatography or other analytical measurements such as high performance liquid chromatography (HPLC) is for sample compounds to be detected contained in a matrix of one or more compounds of high ionic strength. For chromatography, the sample peaks are obscured by the large interfering peak of the sample matrix ion. Also, chromatography is severely changed because the sample matrix ion is of such high concentration that it becomes the major eluting ion, temporarily overriding the eluent.
A membrane suppressor device used in ion chromatography (e.g. of the type set forth in U.S. Pat. No. 4,999,098) also has been used has a pretreatment device on-line with subsequent chromatographic separation using ion chromatography. Pretreatment reduces the concentration of acid or base matrices. This technique is useful for the analysis of anions and cations only when the sample matrix is basic or acidic, respectively. This is because the suppressor device is also an ion exchange device, cation exchange for anion analysis and anion exchange for cation analysis. For example, neutralization of a basic sample matrix for analyzing anions requires the removal of the cationic co-ion to the hydroxide ion and replacing with a hydronium ion to form water for the neutralization reaction. The removal and replacement occur at an ion exchange site of the ion exchange membrane of the pretreatment device.
For some samples, continuous membrane based suppressor pretreatment device may have the required capacity to treat the matrix ion. However, it produces an interfering blank (e.g. sulfate for anion analysis with sulfuric acid as the regenerant). This is due to leakage of the acid regenerant used to supply the continuous source of hydronium ion for the neutralization reaction across the membrane.
Packed ion exchange resin bed columns have been used as pretreatment devices for the same purpose. Packed resin bed suppressors also have an interfering blank for subsequent ion analysis. Moreover, they lack adequate capacity.
A typical sample requiring analysis for trace anions is commercially available sodium hydroxide. The sample can be diluted to a concentration that is compatible with the column capacity, but the required sensitivity for the anions in the original solution is decreased to an unacceptable level due to dilution.